us with a detailed knowledge of the form and relations of all
the animals and plants which have at any period flourished upon
the land-surfaces of the globe or inhabited its waters; it would
enable us to determine precisely their succession in time; and
it would place in our hands an unfailing key to the problems of
evolution. Unfortunately, from causes which will be subsequently
discussed, the palaeontological record is extremely imperfect,
and our knowledge is interrupted by gaps, which not only bear
a large proportion to our solid information, but which in many
cases are of such a nature that we can never hope to fill them
up.

Fossils.--The remains of animals or vegetables which we now find
entombed in the solid rock, and which constitute the working
material of the palaeontologist, are termed "fossils,"[3] or
"petrifactions." In most cases, as can be readily understood,
fossils are the actual hard parts of animals and plants which
were in existence when the rock in which they are now found was
being deposited. Most fossils, therefore, are of the nature of
the shells of shell-fish, the skeletons of coral-zoophytes, the
bones of vertebrate animals, or the wood, bark, or leaves of
plants. All such bodies are more or less of a hard consistence
to begin with, and are capable of resisting decay for a longer
or shorter time--hence the frequency with which they occur in
the fossil condition. Strictly speaking, however, by the term
"fossil" must be understood "any body, _or the traces of the
existence of any body_, whether animal or vegetable, which has
been buried in the earth by natural causes" (Lyell). We shall
find, in fact, that many of the objects which we have to study
as "fossils" have never themselves actually formed parts of any
animal or vegetable, though they are due to the former existence
of such organisms, and indicate what was the nature of these.
Thus the footprints left by birds, or reptiles, or quadrupeds
upon sand or mud, are just as much proofs of the former existence
of these animals as would be bones, feathers, or scales, though
in themselves they are inorganic. Under the head of fossils,
therefore, come the footprints of air-breathing vertebrate animals;
the tracks, trails, and burrows of sea-worms, crustaceans, or
molluscs; the impressions left on the sand by stranded jelly-fishes;
the burrows in stone or wood of certain shell-fish; the "moulds"
or "casts" of shells, corals, and other organic rem